The present invention is directed to applications for improving vision with small- or large-spot scanning refractive laser surgery of the cornea, described in greater detail in Elander, R., Rich, L. F., Robin, J. B.: “Principles and Practice of Refractive Surgery”; W. B. Saunders Company, Philadelphia; 1st ed., 1997 and in Seiler, T.: “Refraktäre Chirurgie der Hornhaut”; Thieme, Stuttgart; 1st ed., 2000. Here the corneal shape is adjusted by an aggregate of hundreds to thousands of laser shots in a pre-calculated ablation profile, according to the intended change of the corneal shape determined in a preceding diagnostic procedure. The temporal sequence of these laser shots is usually defined together with the ablation co-ordinates relative to the eye in a so-called shot-table before the laser treatment. During the surgery procedure the laser spot is positioned using a positioning device, for example a x-y scanner.
So called Eye-trackers (image processing units) are commonly used to measure the exact position of the eye relative to the laser system during this procedure and to provide the eye position as correction information for the laser positioning device to compensate for the patient's eye movements before each successive laser shot is applied as described in FIG. 1.
Starting at the point of time of eye position measurement (exposure time), time is required for image acquisition, data transfer, image processing to yield eye position and adjusting the positioning device to the next ablation position corrected with the eye position obtained (PROCESSING DELAY). This PROCESSING DELAY is normally in the order of tens of milliseconds. With newer high-speed systems this PROCESSING DELAY is reduced to a few milliseconds. A further delay may occur from the time completing adjustments of the positioning device until the laser shot is applied (SYNCHRONIZATION DELAY) due to missing or inaccurate synchronization of the eye tracking with the laser system. This synchronization delay can amount up to an additional period of the sampling interval for the eye tracking i.e. 16.67 ms for 60 Hz sampling of eye tracking (see FIG. 7a).
Together, both delays can lead to considerable positioning errors of laser shots onto the eye due to the patient's eye movements: During fast eye movements (saccades) which exist during the surgery procedure (although the patient is urged to fixate on a point during refractive surgery procedure) a positioning error on the cornea of up to 2 mm can occur with a common overall delay time of 50 ms. Larger errors occur only in a few percent of all laser shots, however they can result in an inaccurate corneal shape, thus imperfect vision correction.
The present invention provides methods and an apparatus for measurement and decrease of these dynamic positioning errors.